National Australia Bank has been the big improver in DBM Consultants' business banking satisfaction survey in recent months, while its big bank rivals have held their ground or slipped back a little.According to DBM's latest Business Financial Services Monitor, NAB's overall rating has increased from 7.1 (out of ten) to 7.2 over the six months to September.Commonwealth Bank has remained steady at 7.5 over the same period, ANZ has been steady at 7.1 and Westpac's rating slipped from 7.5 to 7.4.NAB has the highest rating among medium sized businesses (annual revenue of A$5 million to $50 million).CBA and Westpac share the highest rating among micro businesses (revenue up to $1 million) and small businesses (revenue of $1 million up to $5 million), while CBA has a strong lead in the large business segment (revenue of $50 million and above).DBM director Maria Claridad said NAB's decision to add 200 new business bankers in its small and medium division was the driver of the bank's improved rating.In October last year NAB chief executive Andrew Thorburn conceded that the bank's business banking division's market-leading position was under threat and said the bank was investing in staff and process improvement.He acknowledged that it was hard for commercial customers to do business with the bank and that its processes needed to be streamlined.